The process of producing composite material parts generally comprises various temporally successive stages, with each stage having to be carried out at a given pressure and temperature which may be different from one step to the next.
Consequently, in order to effectively monitor the quality of the performance of a given stage, i.e. the quality of the product at each of the stages leading to the finished material, it is necessary to take these temperature and pressure parameters into account. This is particularly true when the monitoring is done by ultrasound insofar as the ambient temperature and pressure affect the response (sensitivity) of the ultrasonic sensors used.
It is noted that the use of ultrasonic sensors advantageously makes it possible to perform various monitoring functions such as:                Characterizing the fiber preform formed by the flow of a resin injected into a fiber reinforcement (resin matrix/fiber reinforcement);        Determining the advancement of the polymerization of the resin during the polymerization cycle itself;        Determining the fiber volume ratio and/or the local porosity ratio.        
In particular, ultrasound techniques are the only in-depth analysis techniques that make it possible to effectively monitor the absence of porosity in the material produced.
However, ultrasound monitoring is rarely used, at least in so-called “in-situ” fashion, insofar as measurements obtained by means of ultrasonic sensors are difficult to use precisely because of the variability of their sensitivity as a function of temperature and pressure, the levels of which are typically variable in the context of the production of composite material parts.
This is why alternate methods, for example based on dielectrometry (measurements of insulation resistance) or optical diffusion techniques, or systems using fiber optic Bragg gratings are generally used. However, such methods enable only local, non-volumetric measurements at the point where they are positioned, on the surface of or inside the part.
Moreover, even when ultrasonic sensors are used, their use is generally limited to measurements of ultrasound wave propagation times in the medium analyzed (i.e. the part being produced), with no consideration for, in particular, amplitude analysis.